This invention relates to electronic amplifiers, and more particularly to switched-mode radio frequency (RF) power amplifiers.
Transmitters in many modern communication systems, such as cellular radio systems having carrier frequencies of 1-2 gigahertz (GHz) or so, need to have wide bandwidth, wide dynamic range, and high accuracy (low distortion) in phase and envelope to deal with modern modulation schemes that enable effective use of allocated bandwidth. In addition, it is currently preferable that high-performance amplifiers be implemented in CMOS for reasons of cost and integration. Transmitters in battery-powered devices need to be efficient so that battery energy is conserved.
In conventional radio transmitters, the signal information is often represented as two channels in quadrature phase that can be mixed together to form a combined low-power signal that is amplified for transmission. A linear power amplifier is needed for proper amplification of the combined signal, but there is a trade-off between efficiency and linearity in RF power amplifiers. If high linearity is required, a Class A amplifier can be used, but at the cost of low efficiency. If a constant-envelope signal is to be amplified so that linearity is not critical, a high-efficiency switched-mode (Class D, E, or F) amplifier can be used. Switched-mode amplifiers also can provide high power with low peaks in current and voltage, behavior that is important in CMOS implementations due to the limited breakdown voltages of CMOS devices.
Various methods of designing high-efficiency amplifiers are known that require the input signal to be represented in polar coordinates (i.e., as an envelope, or amplitude, component and an associated phase component). It will be understood that polar coordinates are analogous to Cartesian coordinates, and polar modulation (envelope and phase) is analogous to quadrature modulation (in-phase component and quadrature-phase component). Polar modulation can be advantageous because typical active semiconductor devices, e.g., transistors, must operate nonlinearly if they are to operate with high power efficiency. In its nonlinear region, an active device can still represent the phase of an input signal with reasonable accuracy, but not the input signal's envelope. This behavior results in a natural separation of phase and envelope components that enables polar modulation systems to use highly non-linear but highly power-efficient switched-mode power amplifier architectures, such as Classes D, E, and F.
Converting a signal from Cartesian coordinates (in-phase and quadrature components) to polar coordinates (envelope and phase components) is a nonlinear transformation, and so an input signal, e.g., a signal to be amplified, that has a particular bandwidth before the nonlinear transformation will have a much wider bandwidth after the transformation. Modern communication systems usually allow for that by having an internal bandwidth that is four to eight times that of the original signal that needs to be handled in order not to introduce too much distortion. For example, a transmitter presented with an input signal having a bandwidth of 1 megahertz (MHz) usually has an internal bandwidth of at least 4-8 MHz if the input signal is converted from Cartesian to polar coordinates.
Wider internal bandwidths require, among other things, fast digital-to-analog (D/A) converters (assuming a digital input signal) that are harder to design and that dissipate more power. Another common problem with polar modulation is the difficulty of synchronizing the phase and envelope component signals, which is to say that it can be difficult to match the time delays of both component signal paths through the amplifier or transmitter.
In addition, to amplify an envelope component properly, a switched-mode amplifier typically needs some kind of linearization, such as pulse-width modulation (PWM), that itself can be linearized by using low-frequency feedback. PWM is described in U.S. patent application Ser. No. 12/127,126 filed on May 27, 2008, by C. Bryant for “Pulse-Width Modulator Methods and Apparatus”, and linearization and feedback is described in M. Nielsen and T. Larsen, “An RF Pulse Width Modulator for Switch-Mode Power Amplification of Varying Envelope Signals”, Aalborg University, Silicon Monolithic Integrated Circuits in RF Systems, 2007, pp. 277-280 (2007); and International Publication WO 2008/002225 A1 by H. Sjöland for “Switched Mode Power Amplification”. An example of a high-efficiency amplifier that includes band-pass (BP) PWM is described in F. Raab, “Radio Frequency Pulsewidth Modulation”, IEEE Trans. Comm. pp. 958-966 (August 1973). Instead of low-pass filtering the output signal to extract information at the same frequency as the input signal to an amplifier, Raab describes band-pass filtering in a transmitter to extract information around the PWM switching frequency.
Of course, it is desirable to avoid such complications and still have RF power amplifiers, transmitters, and other devices that meet the linearity and power-efficiency requirements of modern communication systems, such as recent- and future-generation cellular radio communication systems.